This application is based on an application No. 2002-002207 filed in Japan, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a negatively chargeable toner used in an image-forming apparatus such as a copy machine, a printer, a facsimile or the like, and to an image-forming method using such a toner.
2. Description of the Related Art
Toner utilized in image-forming apparatuses using electrophotography or the like has conventionally been manufactured according to a kneading and pulverizing method wherein materials such as a binder resin, a coloring agent, a charge controlling agent, if necessary, an offset prevention agent are melted and kneaded, and the resultant mixture is pulverized and classified.
In recent years the demand for image-forming apparatuses that can produce higher quality images has increased and a toner having small particle size and having a sharp particle-size-distribution, which enable higher image quality, has come to be required. In addition, in an image-forming apparatus for forming a full color image using toners of a plurality of colors, a fixing member such as a heat roller and a pressure roller is applied with or impregnated with fixing oil such as silicone oil. Thereby, an offset phenomenon is prevented and the property of release of recording paper from the fixing member is secured. However, the demand for miniaturization and for cost reduction of full color image-forming apparatuses has increased and an oilless fixing apparatus wherein a fixing oil application mechanism etc. is not necessary has been adopted.
In the case of the manufacture of a toner according to the conventional kneading and pulverizing method, there is a problem wherein manufacturing costs increase as the particle size of the toner becomes smaller and as the distribution of particle size becomes sharper. In addition, though it is necessary to include an offset prevention agent in the toner in order to be applied to oilless fixing, waxes conventionally utilized as offset prevention agents, such as polyethylene wax, polypropylene wax, paraffin wax, carnauba wax and ester wax, are not compatible with a binder resin. There is a problem wherein it is difficult to add a large amount of an offset prevention agent to the toner because the offset prevention agent is isolated at the time of pulverization when a toner is manufactured according to the kneading and pulverizing method.
On the other hand, a manufacturing method for toner using a wet granulation method, such as a suspension polymerization method, a dispersion polymerization method, a resin particle association method or an emulsification dispersion method, has been focused on as a manufacturing method that can provide, at a low cost, a toner with a small particle size and a sharp distribution of particle size. Such wet granulation methods do not require a pulverizing process, such as the kneading and pulverizing method, They have such an advantage that a large amount of offset prevention agent can be added to the toner so as to be applied to oilless fixing.
However, in the case that a toner is manufactured according to a wet granulation method, granulation is, in general, carried out in an aqueous medium and a variety of agents, such as a surfactant, a dispersion stabilization agent and a salting agent are utilized. Though the above described variety of agents and water are removed from the toner through cleansing and drying during the manufacturing process for the toner, it is difficult to completely remove them and, due to the effects of these residual components, there is a problem wherein a toner gained according to a wet granulation method (hereinafter referred to as wet granulated toner) is low in chargeability and has an inferior charge stability in regard to environmental change in comparison with a toner gained according to the kneading and pulverizing method (hereinafter referred to as pulverized toner).
In more particular, when a wet granulated toner and a carrier are mixed and stirred for a long period of time, a problem arises wherein the amount of charge given to the toner is reduced in comparison with the initial condition of the toner. When image formation is repeatedly made, a problem arises wherein scattering of toner and image fogging increase.
In addition, in the case that a toner image formed on a photosensitive member, which is a electrostatic latent image-support member, is transferred to a medium to be transferred using a transfer roller, pressure is applied to the toner image on the photosensitive member by means of the transfer roller. There is a problem of the partial adhesion between the toner image and the photosensitive member, resulting in that the toner is prevented from shifting to the medium to be transferred to form image lacking (hollow defects). In general, though a wet granulated toner is uniform in form and has a smooth surface in comparison with a pulverized toner, adhesion of the toner to the photosensitive member is strong so that the problem of hollow defects at the time of transfer becomes significant. In particular, in a full color image-forming apparatus wherein an intermediate transfer member is adopted, a full color image is formed through primary transfer wherein a toner image formed on a photosensitive member, which is an electrostatic latent image-support member, is transferred to a intermediate transfer member using a transfer roller, and through secondary transfer wherein a full color toner image, formed of toner images of the respective colors overlapped on the intermediate transfer member, is transferred to recording paper using a transfer roller. In such a full color image-forming apparatus, as the primary transfer of a toner image on a photosensitive member, as well as secondary transfer of a full color toner image on a intermediate transfer member, is carried, it is further necessary to solve the problem of hollow defects.
A charge control agent (CCA) is added to the toner in order to control chargeability of toner and many CCAs excellent in chargeability include a metal, such as chrome, zinc or cobalt, and are expensive. A CCA-free toner, which does not include a CCA, is desired from the point of view of environmental protection as well as reduction in cost of the toner. However, as a wet granulated toner is low in chargeability in comparison with a pulverized toner as described above, it is difficult to produce a toner containing no CCA.
The present invention is to provide a negatively chargeable toner having excellent chargeablity and stability of chargeability even under environmental change by means of a wet granulation method.
Another purpose of the present invention is to provide a negatively chargeable toner having no problem of hollow defects in copied images at the time of pressure transfer without affecting adversely the chargeability.
Still another purpose of the present invention is to provide a negatively chargeable toner excellent in chargeability even in the absence of a CCA.
The present invention relates to a negatively chargeable toner, comprising:
toner particles prepared by means of a wet granulation method; and
external additives externally added to the toner particles, wherein the external additives comprises a first hydrophobic silica having a mean primary particle size of 5 nm to 18 nm, a second hydrophobic silica having a mean primary particle size of 18 nm to 50 nm, which is greater than that of the first hydrophobic silica, and a hydrophobic titanium oxide having a mean primary particle size of 10 nm to 40 nm, and a full color image-forming method using the negatively chargeable toner.